1. Field of the Invention
The present invention relates to an organic-inorganic composite material, an optical element for use in an imaging optical system such as a camera, and a multilayer diffractive optical element.
2. Description of the Related Art
In conventional diffractive optical systems making use of light diffraction, chromatic aberration is reduced in such a manner that lenses made of lens materials having different dispersion properties are used in combination. For example, the objective lens of a telescope uses a positive lens made of a lens material with low dispersion and a negative lens made of a lens material with high dispersion in combination to correct axial chromatic aberration. However, it is difficult to correct chromatic aberration in the case where the combination or number of lenses is limited or in the case where a lens material used is limited.
U.S. Pat. No. 5,847,877 discloses that high diffraction efficiency is achieved over a wide wavelength range by optimizing the refractive index dispersion of each of optical elements and the shape of a grating formed at the interface between the optical elements. Light flux at wavelengths used is concentrated on a specific order (hereinafter referred to as the design order), whereby the intensity of diffracted light with a diffraction order other than the design order is kept low and the occurrence of a flare is suppressed.
The inventors have investigate optical materials, which are commercially available or under development, for diffractive optical elements and have obtained such distributions as shown in FIGS. 2A and 2B. FIG. 2A is a graph illustrating the distribution of the Abbe number (νd) and refractive index (nd) of common optical materials. FIG. 2B is a graph illustrating the distribution of the Abbe number (νd) and anomalous dispersion (θg, F) of common optical materials. A material for a multilayer diffractive optical element disclosed in U.S. Pat. No. 5,847,877 is included in the distribution shown in FIG. 2A or 2B.
Furthermore, U.S. Pat. No. 5,847,877 discloses that a diffractive optical element made of a material with relatively low refractive index dispersion and a diffractive optical element made of a material with relatively high refractive index dispersion are used in combination for the purpose of obtaining a configuration having high diffraction efficiency over a wide wavelength range.
The larger the difference in refractive index dispersion between a material with high refractive index dispersion and a material with low refractive index dispersion is, the higher the diffraction efficiency of an optical element containing the materials is and the wider the field angle of the optical element is. Thus, in order to accurately correct chromatic aberration, it is necessary to use a material with higher refractive index dispersion (a smaller Abbe number) and a material with lower refractive index dispersion (a larger Abbe number) in combination.
U.S. Pat. No. 6,912,092 discloses an optical material. The relationship between the refractive index (nd) and Abbe number (νd) of the optical material satisfies the inequality nd>−6.667×10−3νd+1.70. The relationship between the anomalous dispersion (θg, F) and Abbe number (νd) of the optical material satisfies the inequality θg, F≦−2νd×10−3+0.59. Since these inequalities are satisfied, increased diffraction efficiency can be achieved over the visible range.
Examples of the optical material disclosed in U.S. Pat. No. 6,912,092 include transparent conductive metal oxides, such as ITO, ATO, and SnO2, exhibiting high refractive index dispersion and low anomalous dispersion.
U.S. Pat. No. 7,663,803 discloses the use of the following diffractive optical elements in combination: a diffractive optical element made of a material which has high refractive index dispersion and which contains a resin such as an acrylic resin or a fluoroacrylic resin and fine particles of a metal oxide such as ITO and a diffractive optical element made of a material which has low refractive index dispersion and which contains such a resin and fine particles of a metal oxide such as ZrO2.
The typical configuration of a multilayer diffractive optical element 201 disclosed in U.S. Pat. No. 7,663,803 is described with reference to FIGS. 3A and 3B. FIG. 3A is a schematic top view of the multilayer diffractive optical element 201 and FIG. 3B is a schematic bottom view thereof. The multilayer diffractive optical element 201 has a configuration in which a high-refractive index, low-dispersion layer 203 having a grating shape and a low-refractive index, high-dispersion layer 204 are deposited on a transparent substrate layer 202 made of glass or plastic with no space therebetween. The order of deposition of the high-refractive index, low-dispersion layer 203 and the low-refractive index, high-dispersion layer 204 may be reversed. Both surfaces of the transparent substrate layer 202 may be flat, spherical, or aspherical. Each of the high-refractive index, low-dispersion layer 203 and the low-refractive index, high-dispersion layer 204 may be sandwiched between transparent substrate layers. Herein, d is the grating height and f is a portion (hereinafter referred to as the base film) other than a diffraction grating.
The following example is disclosed: an example of using a material containing fine particles of a metal oxide, represented by ITO, having high refractive index dispersion to form a diffractive optical element for the purpose of achieving increased diffraction efficiency over the visible range as described above. In general, the metal oxide fine particles are known to have a large absorption in the visible range. Therefore, in the case of using the material in optical elements for imaging devices such as cameras, the content of the metal oxide fine particles in the material may be low to achieve increased transmittance.
However, according to U.S. Pat. Nos. 6,912,092 and 7,663,803, the higher the content of the metal oxide fine particles in the material is, the higher the refractive index dispersion of the material is and the lower the anomalous dispersion thereof is. This allows the diffractive optical element to have reduced grating height and increased diffraction efficiency. In the diffractive optical element, an increase in grating height and a reduction in diffraction efficiency are causes of various flares; hence, the reduction in content of the metal oxide fine particles may not be preferred. Therefore, in order to achieve increased transmittance using the metal oxide fine particles, such as ITO particles, having high refractive index dispersion as disclosed in U.S. Pat. Nos. 6,912,092 and 7,663,803, for example, the base film f may be formed so as to have a small thickness. However, a material forming the diffractive optical element is an energy-curable resin, associated with curing shrinkage, containing a functional group such as an acrylic group. Therefore, the reduction in thickness of the base film f with respect to the grating height d is likely to cause defects such as sink marks. In order to avoid the defects, there are a technique for controlling a curing reaction such that the curing reaction proceeds very slowly and a technique for carrying out a curing reaction using a press mechanism, which leads to the need to increase the takt time for forming or the need to upgrade an apparatus. The techniques are limited in suppressing the defects, such as sink marks.
Thus, in conventional techniques, the following material has been unavailable: a material capable of achieving an optical element which has high diffraction efficiency in the visible range and reduced grating height and which is highly transparent without extremely reducing the thickness of the base film f with respect to the grating height d.